In California, and increasingly in other earthquake-prone states, laws requiring that certain types of equipment installed in hospitals, schools and other public buildings be protected from earthquake caused damage are prevalent. Vibration isolated equipment often is accorded special legal scrutiny.
All equipment with rotating or reciprocating masses or oscillating magnetic fields exhibits vibratory forces which are frequently small enough to allow rigid attachment of such equipment to structures without consequent problems. If the structure containing the equipment is subjected to earthquake induced motions the rigidly attached equipment is normally considered to be protected from earthquake induced damage because said equipment moves in concert with the structure without falling, walking or colliding, assuming the attachments, floors and equipment are structurally adequate. But equipment in many installations is deemed to create excessive vibratory forces while operating if it is rigidly attached to its support surface. It is common practice to support excessively shaky, rigidly framed, equipment on resilient devices to cushion the structure from the vibratory forces and thereby enable day-to-day operation of otherwise unacceptable equipment. Unfortunately this equipment supported by resilient devices is vulnerable to earthquake-induced damage unless motion inhibiting devices are utilized. Common practice is to limit motion of resiliently supported equipment with stops anchored to the floor to limit equipment motion without actually touching the equipment during day-to-day operation. Earthquake. induced motions are thus minimized without compromising day-to-day equipment operation.
There are two different approaches to earthquake protection of resiliently supported equipment, the first of which aims to restrain equipment motion with devices acting directly upon a structurally adequate member of the equipment at locations remote from the load support points. However, such remote restraints are denied the benefit of equipment weight, require added anchor points, are more likely to be jammed since additional anchor devices naturally require more care to align properly. The added anchors cannot be at the structurally more desirable locations at the equipment corners since the corners are normally assigned to the resilient load supporting devices. The unavailable corner locations are also the most desired motion limiting locations since equipment motions are greatest there. In addition locations other than the corners are denied obvious proof of sufficient structural integrety to provide at least one times the force of gravity upward at the contact points that accrues when load support and contact restraint points are combined at the same locations. The resulting appearance of limiting devices remote from load support points is unprofessional, presenting a "scabbed on" aspect, as if restraint were added as an afterthrought. The described first approach results in high costs because of inefficient utilization of materials by having duplication of elements, which is avoidable.
The instant invention, by using a second approach, obviates these difficulties and objections by restraining motion with devices designed to support and restrain in complete integrated support and restraint apparatus units taking advantage of the weight of the supported load to minimize "pullout" loads on the anchors holding the combined device to the support surface.
While there are a number of existing designs featuring earthquake protection and vibration isolation integrated in a single device, some lack totally desired shock cushioning while others lack sufficient contact surface area for proper bearing pressures to avoid cutting shock cushions during an earthquake. Some existing devices have contact surfaces for the restraints high above the floor or other load support surface, causing undesirably high pullout loads on floor anchors. Conventional protective devices for equipment hide the shock cushions that do exist behind immovable structure members, thus preventing shock cushion inspection without the complete disassembly or removal of the protective device, while other devices so position the cushions that service again requires complete removal and disassembly of the device.
Like an untested fire hose in a building awaiting its call to serve, earthquake protection equipment is virtually untested until called upon to perform in the undeterminable future. Thus, proper design should feature ease of inspection, accessibility and of replacement of aged, cracked or hardened shock cushions, which have an average life of ten years.